1. Field of the Invention
The present invention relates to a method on incinerating substances such as municipal wastes and industrial wastes (called "refuse" hereinafter) while fluidizing them in a fluidized bed. More particularly, the invention pertains to a method of denitrification without using catalysts in such refuse incinerators (called "catalystless denitrification" hereinafter) that is capable of decreasing the amount of nitrogen oxides (called "NOx" hereinafter) present in the combustion exhaust gas generated while incinerating the refuse in a fluidized bed.
2. Description of the Prior Art
Fluidized bed incinerators for disposing of refuse by incineration are known. The method of disposing of refuse in such a fluidized bed incinerator involves burning the refuse while fluidizing it with air, wherein a fluidizing medium such as sand (called "fluidizing medium" hereinafter) that aids in improving fluidization and combustion of refuse is fed to the bed along with the refuse.
Generally, fluidized bed incinerators are equipped with a plurality of air diffuser tubes or plates (called "air diffusers" hereinafter) in the lower part of the fluidized bed incinerator body (called "furnace body" hereinafter), as well as a refuse feeding mechanism and a fluidizing medium feeding mechanism in the upper part thereof.
The refuse and the fluidizing medium deposited onto the air diffuser tubes are fluidized by primary air blow from the air diffusers, and as they are fluidized, the refuse is burned.
The refuse may contain low calory refuse such as food discards or high calory refuse such as plastics. The refuse may comprise shredded paper or chipped furniture, fragmented metallic or vitreous containers, bottles, and cans, and other sundry substances. As the refuse is fed to the fluidized bed, the combustible portions thereof are burned. Refuse substances such as plastics undergo pyrolysis and therefore generate various pyrolysis, or thermal decomposition gases, while the incombustible portions such as metals and glasses are left unburned (called "combustion residue" hereinafter).
In the fluidized bed, a moving bed of the fluidizing medium is formed, and the medium particles descend as the feeding of the fluidizing medium continues. As a result, while the combustibles are burned or decomposed within the bed, the combustion residue is drawn downwardly along with the fluidizing medium and are removed from the furnace body through gaps between the air diffusers which are located in the lower part of the bed, where the fluidizing medium is separated from the combustion residue to allow the fluidizing medium to be recirculated back to the fluidized bed.
Secondary air is supplied to the freeboard part of the furnace body, i.e. that portion of the furnace body which extends above and over the fluidized bed (called "freeboard" hereinafter), wherein the generated pyrolysis gases are burned with the secondary air.
Since the fluidizing medium, e.g. sand, oscillates while it descends in the bed and is heated, it promotes agitation and dispersion of the refuse. Therefore, the refuse fed to the fluidized bed becomes uniformly dispersed under the presence of the fluidizing medium, and is dried, ignited, decomposed, and burned instantly. The airborne ash and dust generated in the furnace body are withdrawn from the upper part of the incinerator and are collected in an electric precipitator.
Thus, the refuse introduced into the fluidized bed is almost completely disposed of with the exception of metallic, vitreous, or ceramic residue, which is generally 2% of the refuse; this means that 98% of the refuse can be disposed of by a fluidized bed incinerator. That the combustion residue is only 1/3 of that of a conventional mechanical incinerator such as the stroker type combustor is a primary advantage of the fluidized bed incinerator.
As shown in FIG. 3 of the accompanying drawings, however, some 100 ppm of NOx is contained in the combustion gas exhausted from fluidized beds. The prior art method of decreasing NOx consisted of leading the exhaust to a denitrification apparatus in which the NOx is removed, but this approach necessitates substantial additional equipment and results in the incinerator plant being large and complex.